The present disclosure relates to wideband piezoelectric transducers for harmonic imaging. More particularly, the disclosure relates to wideband piezoelectric single crystal transducers which can be used to cover fundamental to at least third harmonic frequencies.
Ultrasonic imaging has quickly replaced conventional X-rays in many clinical applications because of its image quality, safety, and low cost. Ultrasonic images are typically formed through use of phased or linear array transducers which are capable of transmitting and receiving pressure waves directed into a medium such as the human body. Such transducers normally comprise multielement piezoelectric materials which vibrate in response to an applied voltage to produce the desired pressure waves.
To obtain high quality images, the transducer must be constructed so as to produce specified frequencies of pressure waves. Generally speaking, low frequency pressure waves provide deep penetration into the medium (e.g., the body), but produce poor resolution images due to the length of the transmitted wavelengths. On the other hand, high frequency pressure waves provide high resolution, but with poor penetration. Accordingly, the selection of a transmitting frequency has involved balancing resolution and penetration concerns. Unfortunately, resolution has suffered at the expense of deeper penetration and vice versa. Traditionally, the frequency selection problem has been addressed by selecting the highest imaging frequency which offers adequate penetration for a given application. For example, in adult cardiac imaging, frequencies in the 2 MHz to 3 MHz range are typically selected in order to penetrate the chest wall.
Recently, new methods have been studied in an effort to obtain both high resolution and deep penetration. One such method is known as harmonic imaging. Harmonic imaging is grounded on the phenomenon that objects, such as human tissues, develop and return their own non-fundamental frequencies, i.e., harmonics of the fundamental frequency. Due to this fact, and to the increased image processing capabilities of digital technology, it is possible to excite the object to be imaged by transmitting at a low (and therefore deeply penetrating) fundamental frequency (fo) and receiving at a harmonic wave having a higher frequency (e.g., 2fo) that can be used to form a high resolution image of the object. By way of example, a wave having a requency less than 2 MHz can be transmitted into the human body and one or more harmonic waves having frequencies greater than 3 MHz can be received to form the image. By imaging in this manner, deep penetration can be achieved without a concomitant loss of image resolution.
Harmonic imaging can also be particularly effective when used in conjunction with contrast agents. In contrast harmonic imaging, gas or fluid filled micro-sphere contrast agents are typically injected into a medium, normally the bloodstream. Because of their strong nonlinear characteristics when insonified at particular frequencies, contrast agent resonation can be easily detected. Therefore, injection of contrast agents into the body can enhance the imaging capability in the detection of blood-filled structures and blood flow velocity in the arterial system. For instance, contrast harmonic imaging is especially effective in detecting myocardial boundaries, assessing microvascular blood flow, and detecting myocardial perfusion.
Despite the advantages possible with harmonic imaging (both tissue and contrast), serious limitations to its utilization exist. In particular, due to the need for transmitting and receiving both high and low frequency waves when performing harmonic imaging, the transducer used must have a very large bandwidth. In different applications, multiple second harmonic frequency selections may be required to obtain acceptable penetration and resolution. Similarly, a particular contrast agent may resonate better at a specific imaging frequency.
Obtaining wide bandwidths from the small piezoelectric elements currently used in phased array transducers is particularly difficult. Present day transducers are typically made of lead zirconate titanate (PZT) based ceramics. Such transducers typically have a xe2x88x926 dB bandwidth of 55% to 85%. Unfortunately, this bandwidth is barely enough to cover the frequency range of the fundamental and second harmonic and therefore the harmonic performance is limited. A wider bandwidth transducer which covers multiple pair of second harmonics and even the third harmonic is required to improve harmonic imaging.
To cite a specific example, transesophageal probes create challenges in conducting harmonic imaging due to the bandwidth limit. Since there is no chest wall attenuation, transesophageal probes typically operate at higher frequencies for better resolution, typical 5-7 MHz. Harmonic study requires the probe to be operated at a much lower frequency to burst contrast agents, typically 2-3 MHz. The required tissue harmonic imaging frequencies are also much lower than the high resolution imaging frequencies. The PZT-type transducers cannot offer the bandwidth to perform both harmonic and high resolution imaging from one transducer. This has prevented the use of transesophageal probes in tissue and contrast harmonic imaging applications.
Recently, vastly improved electromechanical properties have been observed with single crystals of Pb(Zn1/3Nb2/3O3xe2x80x94PbTiO3) (PZN-PT) and Pb(Mg1/3Nb2/3)O3xe2x80x94PbTiO3 (PMN-PT). Using these materials, longitudinal coupling constants, k33, of 85% to 93% have been observed as compared with conventional PZT-type ceramics which normally exhibit a k33 value of approximately 70% to 75%. As known in the art, the coupling constant, k33, represents the efficiency of conversion of electrical energy to mechanical energy and vice versa. This high coupling of PZN-PT and PMN-PT single crystals provides the potential for improved sensitivity and bandwidth in transducer design.
Obstacles to the use of PZN-PT and PMN-PT single crystals still exist despite the greatly improved performance they can provide. For instance, although the single crystals have relatively high free dielectric constant, the clamped dielectric constant, Ks, is very low (e.g., Ks=800-1400) for compositions near the morphotropic phase boundary (MPB). For phased array transducers with small element sizes, the impedance of single crystal transducers is relatively high and this may cause electrical mismatch between the transducer and the system transmitter which reduces the sensitivity and bandwidth of the single crystal transducers.
Although lead-based single crystals provide high coupling, they also have high acoustic impedance. Effectively coupling the acoustic energy from single crystals with high acoustic impedance into the medium with low acoustic impedance is also critical for achieving broad bandwidth.
Another obstacle to the use of PZN-PT and PMN-PT materials is the temperature stability of the materials. Specifically, these materials have lower Curie temperatures at the rhombohedral phase compositions in comparison to the typical PZT-type ceramics used for medical imaging applications. The phase transition between rhombohedral to tetragonal occurs at an even lower temperature. Therefore, these materials are more susceptible to depoling during use.
It can, therefore, be appreciated that it would be desirable to have a piezoelectric transducer which possesses both a very wide bandwidth and high sensitivity so as to be well-suited for imaging of fundamental to at least third harmonic frequencies. Furthermore, it would be desirable to have such a transducer which also avoids the problems discussed in the foregoing.
The present disclosure relates to an ultrasonic imaging system for harmonic imaging of an object in a medium. The system comprises a transducer formed of a single crystal of a piezoelectric material, a transmitter which causes the transducer to transmit fundamental ultrasonic signals into the medium, a receiver for receiving harmonic ultrasonic signals from the object in the medium, and a control system electrically connected to the transmitter and the receiver which is used control the operation of the transmitter and the receiver.
In a preferred embodiment, the single crystal of piezoelectric material comprises a PMN-PT material or a PZN-PT material. Through use of this material, xe2x88x926 dB bandwidths of at least approximately 95% are obtainable.
In addition, the present disclosure relates to an ultrasonic imaging method for imaging an object. The method comprises the steps of causing a transducer formed of a single crystal of piezoelectric material to emit an ultrasound signal at a fundamental frequency, receiving first echoes at a harmonic frequency of the fundamental frequency, receiving second echoes at another harmonic frequency of the fundamental frequency, and processing the first and second echoes to produce an image of the object.
In a preferred embodiment, at least the third harmonic frequency is received and used for imaging. In another embodiment, at least two of the harmonic frequency signals can be received separately at the same time, and combined to form a hybrid image. Alternatively, first and second fundamental frequencies can be transmitted simultaneously such that the second harmonic frequency of one is substantially the same frequency of the third harmonic frequency of the other. In addition, the second and third harmonic signals of the transmitted echoes can be received simultaneously to form a mixed image.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.